


Results (or What Happens When You Give Slim What he Wants)

by SoloShikigami



Category: Undertale (Video Game)
Genre: Alternate Universe, M/M, Mating Cycles/In Heat, Mpreg
Language: English
Status: Completed
Published: 2020-02-07
Updated: 2020-02-07
Packaged: 2021-02-27 22:00:51
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,532
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22592926
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Originally posted in 2017.When Slim arrives in the Underswap's realm, old habits die hard - he follows Swap around like a lost puppy, already the town thinks they are in a relationship where Swap is the Dominant one. When Slim catches Swap in a private moment, one thing leads to another, revealing feeling that had already existed, and soon Swap realizes that while the townsfolk may have their roles switched, their perceptions weren't wrong.
Relationships: Papyrus/Papyrus
Comments: 5
Kudos: 95





	Results (or What Happens When You Give Slim What he Wants)

**Author's Note:**

> Originally posted on Tumblr on 2017 as a fic for Silverwolf715!

"Mmm, shit, Pap, you're so damn tight today," Slim purred, not letting up as he slammed into the skeleton beneath him. "Were you hoping for one of your breaks to go like this? It has been a couple of days, after all."

Swap couldn't reply past a few whimpers, his voice muffled by the sheets his face was pressed into, his arms tangled up in the sleeves of his hoodie; Slim had used it to tie his arms together behind his back.

"Nyeh, heh, well, personally, I'm glad it did. I just couldn't help myself, you know? You're so fucking sexy, you turn me on so much, I just had to have you," Slim continued in a murmur just loud enough for Swap to hear, bending over to lick up his upper spine, making the other's entire body convulse suddenly.

Slim's eyes widened a little in a mix of surprise and amusement and he reached underneath to find slippery magic dripping from where they were joined. He chuckled, gathering some on his fingertips.

“Ohhh, stars, I felt that," Slim murmured, taking his slicked fingers and tracing them around Swap' tailbone, drawing out a loud cry that Swap tried to quiet. "Your brother isn't home, darling, so you can be as loud as you want. I prefer you to be as loud as you possibly can."

Swap continued to moan loudly as Slim continued to roughly fuck him.

\-----

One of the things that Swap found oddly appealing about his relationship with Slim was how everyone perceived them. Ever since Slim appeared in their world as a bruised, broken, mental, emotional, and physical mess of a monster, the two skeleton brothers had dedicated themselves to healing him.

It took a little while for Slim to adjust to how this world worked, more notably it took a few weeks for Swap to get him to stop referring to him as "sir" and to stop calling Blue "m'lord." While his younger brother was amused by the title, Swap could see that it was slowly getting to his brother's head and was thankful when Slim finally stopped.

Swap loved his brother, but he would be damned if he was going to give him a title like that.

The one habit that he couldn't seem to get Slim out of was the way he would follow him around like a puppy. He would always walk behind him, or if he did walk beside him, he would still be half a step behind or grab onto his arm nervously. Slim even went so far as to ask Swap if he could be led by a leash, but that was where Swap drew the line. Slim continued to stutter in front of the rest of the town, apologized (and overly so) for any and every possible thing, and was still seen as a weak, submissive mess of a monster. The rest of the townspeople were sympathetic, always smiling at him and assuring him that all was well, and he would give a small, sheepish smile and blush before running off to hide behind Swap.

Slim would go to work with him, sit on the ground next to him whether it was at the snowy Snowdin sentry station, the slightly wet rock next to his telescope station in Waterfall, or the baked clay of the Hotland hot dog stand. He would often lean his head into Swap’s lap and just as often he would doze off, which suited Swap just fine as he usually dozed off, too.

Already all of these behaviors had the town thinking that they were in some sort of relationship (albeit perhaps a tad unconventional) and the rumors had set Swap as the dominant one in the relationship. Even Blue thought something was going on between the two of them, as Slim also slept in Swap’s bed despite the fact they had a perfectly nice couch for him to sleep on, and Blue did offer his own bed.

“Brother, are you sure you’re not keeping something from me?” Blue asked one day at breakfast, glancing over at Slim briefly.

“Why would you say something like that?” Swap asked.

Blue smirked. “For one, you’re walking funny.”

“You’re the one who keeps calling me a lazybones, guess my leg bones decided to doze off.”

“Pff, Papyrus, that was the weakest attempt at a joke I have ever heard!”

“Hmm, tough to be on a roll if I’m not on the ball, eh bro?”

“Gah! That was horrendous!”

“You trying to butter me up?”

“Better, but still horrible!”

Slim quietly snickered, not looking up from his bowl of cereal.

That was how it went until Blue finally stopped asking, but of course he wasn’t ignorant. His brother may not come out and say it, but he knew that there was something going on between him and this odd copy from another universe.

But then one day, it all got flipped on its head.

Slim accidentally walked in on Swap while he was masturbating.

"Dude! Don't you know how to knock?!" Swap exclaimed in frustration and embarrassment as he pulled the sheet over his lap.

Slim blinked in the doorway, his eyes riveted to the orange glow that was still obvious under the thin, beige sheet. He slowly dragged his eyes up Swap's body until they rested in the startled one that still stared at him.

Slim stepped in and shut the door with his foot. After a moment of thought, he reached behind him and locked the door.

"W-what are you doing?" Swap asked.

"Since you haven't told me to leave, I'm going to make the assumption that my presence doesn't bother you. Perhaps I could be of assistance?" Swap murmured, slowly walking towards the other skeleton.

Swap frowned slightly. He wasn’t quite sure what to do or how to feel about all of this. He flinched and was snapped out of his thoughts when a weight settled on the mattress near his feet.

“I’m not hearing a no,” Slim said, reaching over with one hand to stroke Swap’s shin. “Are you in heat, or are you just bored?”

“Bored,” Swap murmured.

“Are you saving yourself for anyone?”

“No.”

Slim smiled. “Then allow me to aid you?”

Swap tossed the idea around his head for a moment. He felt that there was likely some sort of ethical dilemma that should be attached to a decision about this, but who knew what was going to happen at the next reset, so, did it really matter? He looked up at Slim again to find the other skeleton was watching him intently and waiting.

Swap had to wonder if he would have someone who was so willing to lend a hand like this.

“All right,” Swap said, his fingers clenching in the sheet.

Slim’s smile widened and he chuckled as he moved onto his knees and reached for the sheet.

“Thank you for allowing me this,” Slim murmured, pulling the sheet away slowly.

Swap’s grip tightened for a split second, but he released the sheet and slowly leaned back on his pillow. Slim nodded in appreciation for Swap’s trust and continued to pull the sheet away, revealing Swap’s body little by little.

Slim nearly groaned once the sheet slipped off completely and lay in a small heap on the floor as his eyes roved over the smooth, white bone of Swap’s legs and pelvis, his spine arching slightly in response to the cooler air of the room, and the decent hard on his magic had formed.

Swap fidgeted under the intense gaze. “Well, you wanted to help; are you just going to stare at it for the rest of the afternoon?” he quipped in an attempt to hide his nervousness.

Slim’s long tongue slipped between his sharpened teeth with a moan, making Swap’s own mouth click shut as a blush brightened his cheekbones. Slim’s gaze turned almost predatory as he moved to lay on his front, pushing Swap’s legs aside, wrapping one arm under one femur and gripping his hipbone while the other gently wrapped around the base of the orange cock.

“Are you ready?” Slim asked.

Swap’s breath turned a little erratic, but he nodded. Slim gave him a reassuring smile, which only made him relax marginally, then Slim’s mouth covered him entirely. Swap’s head dropped back as he let loose a loud cry.

Slim sucked down the cock eagerly, his tongue swirling around it, his hand at the base squeezing and releasing, his other hand and arm kept the one side of Swap pinned down. Swap thrashed beneath him, a hand over his mouth to contain the string of swears and gasps that escaped.

“Fuck, shit, fuck, Slim, fucking hell, man,” the words spilled from Swap’s mouth.

Slim hummed around him, wringing another strangled cry from his double, then began to lightly scrap his teeth over Swap’s shaft.

“Slim, Slim, I’m going… you’ll make me…. Shit, shit SHIT!”

Slim swallowed him down, groaning as the hot spurts of magic hit the back of his throat. It was only when the cock in his mouth stopped twitching that he slowly pulled off, pressing kisses around Swap’s pelvis and femurs before finally glancing up. Swap was breathing heavily, but it looked like he had passed out. Slim crawled over him until they were face to face and he pressed a gentle kiss on his jaw.

When Swap came around, he found that Slim had him wrapped up in his arms, cuddling him.

“Welcome back,” Slim murmured. “Are you all right?”

“Shit, Slim, what the fuck?” Swap said with a sigh, pressing back into his hold. “Where the hell did you learn to do that?”

Slim chuckled. “I am very good at taking care of such things,” he said. “And now that you have granted me permission, it will be my duty to be sure that you are thoroughly satisfied.”

Swap blinked. “What do you mean?”

Slim chuckled and rolled his hips up into Swap’s backside, drawing a gasp from the other skeleton when he realized that Slim was fairly well-endowed.

“I mean that the responsibility of keeping you satisfied now belongs to me,” he rumbled against the back of Swap’s neck. “I will be sure that your needs are met whether you just get a little horny or you go into a full-blown heat. Granted, I will do my best to satisfy you before you even get to that point.”

A small shiver ran up Swap’s spine at the thought.

That night, it seemed that Slim waited until Blue was in bed and likely asleep before he made his way quietly to Swap’s room.

Well, rather, he used one of his shortcuts.

“What the-?” Swap gasped, not thinking that Slim would be in his room, much less by teleportation.

“I’m here to fulfill my promise,” Slim said, beginning to strip down.

Swap frowned a little; he wasn’t planning on having sex that night, but he was a little curious to what Slim had in mind and what he was capable of. He chuckled and shifted over on his mattress, lifting the sheet in invitation.

“All right, then, big boy, if you insist,” Swap said with a chuckle.

Slim gave him a look; it was serious, almost dead-pan, dark orange magic swirling in his eye-sockets. Unlike Swap, both of his still glowed, and it was almost an unsettling vision to see in the dark.

“Strip,” Slim said, the word hanging in the air as it tried to decide if it was a request or a command.

“What?”

“I can assist you, if you want.”

Before Swap could answer, Slim moved forward and kneeled on the bed before reaching don to yank the dark tank top over Swap’s head.

“Hey!” Swap protested, trying to shove the other away as his pants were next. “What do you think you’re doing?”

Swap was stilled when those eyes focused on him again.

“This is my job, this is what you agreed to, this is how I will serve you,” Slim murmured, succeeding in pulling Swap’s pants down his legs and tossing them to the floor with the rest of their clothes. He moved over Swap and pinned him down with his body. “Are you ready?”

Swap shuddered at the feeling of the heavier body above him, finding something very appealing in this moment where he felt helpless to physically do anything and yet knowing it could all stop with a single word. It was an odd sense of having all of the power in the world while holding no power at all. Swap moaned in appreciation as Slim ground his body against him.

"Hmm, how shall I service you," Slim murmured in a thoughtful tone. Before Swap could formulate an answer, Slim's fingers traced up his pubic symphysis slowly, drawing another gasp and Swap’s magic formed. “Oooh, what’s this?”

Swap whimpered as Slim continued to trail his fingers softly around his forming magic. He tried to sit up but Slim made low growl in his chest and pressed his other arm across his collarbones.

“Hush, hush, I’ll take good care of you,” he murmured, pressing down to kiss Swap deeply as he coaxed his magic into the shape he wanted.

Swap’s soul pulsed heavily in his ribcage; there was an instant where his instincts told him to stop this and throw Slim off, but it was soothed quickly by gentle kisses and gentle murmurs from the skeleton pinning him.

“You’re being so good,” Slim said, dropping another kiss. “Good boys get rewarded well.”

Slim shifted his weight while still keeping Swap where he wanted him. Swap cried out when he felt a hot, blunt, hard head slip between his folds, slicking itself on the magic Swap coaxed from him.

“I’ll be gentle, just relax,” Slim promised.

Swap wanted to say something, but the words fled from his mind as Slim slowly sank into him, a strangled cry stuck in his throat as he was breached and filled. Slim moaned.

“Ohhh, damn, you feel so good,” he whispered. “So tight around me, so soft, so wet, such a good boy taking me in so well.”

Tears trickled from Swap’s eye-sockets, overwhelmed with sensation, Slim’s words penetrating his psyche and turning him on more than he thought possible. Finally, whimpered sounds escaped him as Slim was hilted, arms wrapped around Slim’s neck tightly, his body quivering, his hips hitched up slightly, bringing him in even deeper.

“Fuck, you’re enjoying this a lot, aren’t you?” Slim asked, then he chuckled when Swap only gave a small nod. “Good, haah, I am, too, but I can’t control myself much longer,” he licked up the side of Swap’s face. “I’m going to give you all I got.”

That was how it had been; Slim had a near-insatiable sex drive and it was a rare night that Swap wasn’t fondled or fucked into exhaustion. He had even woken up to Slim touching him or sucking him off a few times, but Slim was always so grateful to play with Swap and his body that he had a hard time denying him. Any moment Slim could get Swap alone meant being restrained or pinned down and fucked mercilessly, but Swap found it thrilling and even charming to a certain extent, and he didn’t want to put a stop to it.

\-----

“Brother, are you sure you are all right?” Blue asked, looked at Swap worriedly across the breakfast table.

Slim’s eyes were immediately trained on his double; he wondered how he didn’t see how Swap’s face took on an ashy hue. But Swap was also already downstairs before Slim had woken up.

“Yeah, bro, I’m good,” Swap said, though even he didn’t sound convinced.

“Mweh, well, I don’t know if you-!”

Blue’s sentence was cut off by Swap suddenly standing up and running over to the sink. Both Slim and Blue were on their feet in a second when the sounds of retching came from Swap and they gasped to find the poor monster’s body emptying itself of magic. Blue immediately began rubbing Swap’s back while Slim stood beside them, fretting.

“What do I do? What do I do?!” Slim asked as Swap seemed to be finished and his whole body shook.

“Please get a glass and let’s get some water into him,” Blue said calmly, though his voice was shaking.

With a groan, Swap slid down onto his knees. Slim moved to do as he was told, running the water in the sink for a moment to wash some of the magic away while Blue untied his scarf and used a corner to wipe the sweat from Swap’s forehead. Slim joined them on the floor a moment later, gently tipping the glass of water for Swap to drink.

“I’m okay,” Swap murmured, his voice a little ragged from being ill.

“Like hell you are,” Slim muttered.

“Papyrus, you are ill,” Blue said. “How else have you been feeling? Did you eat some bad honey? I told you not to hide that stuff in your room, it’s so unsanitary!”

Swap shook his head. “No, I didn’t have any honey in the past day,” he said. “I was fine, just… well…”

Swap looked hesitant, but with the other two looking at him so intently, he knew he had to tell them.

“M-my soul has been feeling kind of funny,” Swap said softly.

“Brother, please let me see it,” Blue said with an eerie calm. “I have to make sure it’s not something really, really serious.”

Swap opened his mouth to protest, but Slim put a hand on his shoulder.

“Please?” he said plaintively.

Their eyes met; Swap already knew that Slim was blaming himself for whatever was wrong with him. Maybe their play sessions were too rough, maybe they drained Swap too much, who knew?

Swap sighed. “Okay, you two win.”

He closed his eyes as Blue put a hand just to the left side of his sternum and he did his best to relax as Blue gently and carefully pulled out his soul.

“Oh, oh my,” Blue murmured, cradling the organ in his hands. “P-papyrus?”

Swap opened his eyes slowly, but then they shot wide open when he saw his soul. Slim was also staring, entranced.

In the center of pale amber-colored soul was a tiny, white soul that was about a quarter of the size of Swap’s. It pulsed gently inside, its glow adding to Swap’s.

“What is that?” Slim asked, the grip on Swap’s shoulder tightening.

“Ah, it, i-it’s a-” Blue stammered, tearing his eyes away from what lay in his hands to the two lanky skeletons. “It’s a baby. Brother, you’re carrying a child.”

The two blinked at Blue. Slim’s eyes widened impossibly for a moment as a smile slowly turned his sharp teeth upwards. Blue had to marvel at that; it was the first real, true, genuinely happy smile he had ever seen on the other.

“Swap, oh, it’s beautiful,” Slim whispered, then he let out a loud bark of laughter and gripped Swap in a tight hug. “I’m going to be a Daddy!”

“I guess I’m going to be an uncle,” Blue murmured, returning his attention to the souls in his hands, cradling them even more protectively, not realizing they gave a jolt as if startled. “Papyrus, isn’t this amazing?”

Silence.

“Papyrus?”

Swap had fainted.

\-----

It didn’t take long for the rumors to die down as the suspicions of the town were quickly confirmed, though changed. Slim now walked alongside Swap, one arm protectively around him at all times, looking proud and pleased as Swap began to show the evidence of their involvement with one another.

“Slim, I’m pregnant, not invalid,” Swap grumbled when the other tried to carry him.

“But you’re getting on in your pregnancy and I do not want you to slip!” Slim insisted.

“But what if you slip? Then both of us will get hurt.”

Slim considered, but then conceded the point and they continued to Swap’s outpost in silence.

“I have been meaning to ask you,” Slim said.

“Hm?”

“How did this happen? How did we become parents?”

“You fucked me and knocked me up, that’s how.”

“But how come you didn’t get pregnant sooner?”

Swap sighed and stopped walking. Slim also stopped.

“Swap?”

“Intent,” Swap murmured.

“What do you mean?”

Swap pulled Slim in for a hug, resting his head on the other’s shoulder.

“I love you,” Swap said softly. “I loved you so much one night that I must have wanted to carry a child for you. I see how you respond to the kids in town; you aren’t as shy with them, you make faces to make them smile and giggle, and I guess it must have got to me during my last heat cycle.”

“I, I l-love you too,” Slim murmured, the words feeling foreign but good. “I love you very much.”

Swap smiled. “Good, because that will make raising this kid a lot easier.”

Slim laughed.

“Okay, enough mushy crap,” Swap let Slim go, but gripped his hand and started to walk again. “Oh, wait, one thing?”

“Yes?”

“Stop looking so damn smug about getting me pregnant.”

Slim chuckled. “Yes, dear.”

**Author's Note:**

> Like what you read? Please check out my social media for more fun!  
> [ My Tumblr](https://soloshikigami.tumblr.com/)  
> [ My Twitter](https://twitter.com/SoloShikigami)


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